The manufacture of cellulose paper products typically begins with the wood chip. Though cellulose pulps from which paper is made can be manufactured from a variety of cellulose materials, including grasses, agricultural waste, hemp, sawdust, etc. as well as recycled papers, the predominant form of cellulose that is treated in modern pulp mills is the wood chip. These chips vary in length and width from 25 to 50 mm or more but are typically less than 10 mm in thickness, for example, from 4 to 8 mm in thickness. Wood chips are typically introduced to the pulping process via some form of isolation device or air-lock in order to minimize the escape of heat and gases from the process as the chips are introduced (see U.S. Pat. No. 5,547,546, incorporated by reference herein).
One typical process that chips undergo after they are introduced to the pulping process is steaming. Steaming, that is, the exposure of the chips to steam in a retention vessel, has several functions. The steam begins the heating process that culminates in the chips achieving a pulping temperature of between about 140-180.degree. C. Steaming, more importantly, displaces the air that is naturally present in the cavities within the chip. The removal of the air, or de-aeration, of the chips insures that the chips will not impose a buoyant force, that is, they will not tend to float, during aqueous treatment. Steaming, and the consequent condensation of the steam in the chip, also enhances the penetration or impregnation of cooking chemical into the chip. Since de-aeration and impregnation are critical to the quality of the pulp produced, adequate or proper steaming is essential when producing pulps for the paper markets of the late 20th century and the new millennium.
However, proper steaming of wood chips is not easy to achieve, especially continuously in a retention vessel through which chips pass for a limited amount of time. Wood chips, as described above, can have varied geometries and do not lend themselves to uniform passage through vessels, especially when the chips are discharged via restricted outlets. Special vessel geometries or agitation is typically required to continuously pass chips through cylindrical vessels, for example, the geometries described in U.S. Pat. Nos. 5,500,083; 5,617,975; 5,628,873; 4,958,741; and 5,700,355, and marketed under the name Diamondback.RTM. by Ahistrom Machinery Inc., of Glens Falls, N.Y., which do not require agitation or vibration to uniformly pass wood chips in a "plug flow" regime. Even when handled by vessels or bins having the geometries described in these patents it can still be difficult to expose the chips to steam for sufficient length of time to ensure adequate de-aeration and impregnation.
Typically, chips are introduced to the steaming or retention vessel via a centralized inlet, for example, by an air-lock-type gate located in the top cover of a cylindrical vessel. The chips thus typically fall along the centerline of the vessel and form a conical pile in the middle of the vessel. Depending upon the character of the chips, the angle of this conical pile, that is, the angle of repose, is typically about 40-50.degree.. The steam with which the chips are treated is typically introduced via one or more nozzles uniformly distributed about the circumference of the vessel and at an elevation below the conical chip pile, that is, at a point below where the chip pile touches the vessel wall. The steam then passes upward through the chip mass to heat, de-aerate and impregnate the chips. This heating, deaeration and impregnation typically requires a minimum retention time to allow the steam to diffuse into the chips and the steam to condense in the chips. This retention time is typically defined by the distance between the steam inlet nozzles and the elevation at which the width of the bin is completely filled with chips. Above this point, the steam will exit the chip pile by following the path of least resistance. When the chip pile is conical in shape, this path is typically not through the conical pile above but undesirably through a side of the conical pile. Thus, some of the chips in the conical pile are not exposed to steam as long as desired. The steam that passes through the chip pile exits the bin through an outlet in the top cover of the bin. Thus, the conical pile of chips at the top of the bin typically is not as uniformly exposed to steam as the chips below the conical pile. If possible, it is therefore desirable to reduce the height of the conical chip pile as much as possible to minimize the time that the chips are not exposed to steam. Since the chips are typically introduced through a central inlet, it is difficult to change the geometry of the chip pile without imposing some external mechanism for distribution of the chips across the bin cross-section.
As the chips are introduced to the vessel, it is also desirable to provide the most uniform distribution of chips such that a uniform downward load is exerted on the chip mass below the top of the chip pile. An uneven distribution of chips can produce an uneven load on the chip pile which can promote a non-uniform movement of the chip mass below. In the worst case, an uneven chip load can cause a flow regime referred to as "rat-holing" in which the flow is limited to localized regions and the flow stagnates elsewhere. This undesirable flow regime can result in non-uniform treatment of the chips in the vessel and in extreme instances can result in a plugged vessel having no material flow.
Another desirable feature of an air-lock or a device for introducing wood chips, or other comminuted cellulosic fibrous material, to a vessel is to have the chips introduced along the centerline or axis of the vessel such that a uniform, for example, conical, chip pile is produced. In other words, it is desirable to produce a pile of material in the vessel that is axially symmetric such that little or no non-uniform loading is produced which can produce non-uniform flow and non-uniform treatment of the material in the vessel. Though the synchronized chip gates of PCT publication WO 96/17124 promote this uniform feeding of chips, this prior art, having only two gates, has a limited capability of providing the desired distribution. The multiple gate embodiment of the present invention, for example, having three or more controllable gates, provides a better mechanism for establishing a chip pile in the vessel which is more axially symmetric about the axis of the vessel and which provides a more uniform load on the chip mass below.
According to one aspect of the present invention there is provided a method of treating comminuted cellulosic fibrous material, including by controlling the top profile of comminuted cellulosic fibrous material established in a treatment vessel having an isolation device at or adjacent the top of the treatment vessel, comprising: a) Causing comminuted cellulosic fibrous material to flow downwardly through the isolation device into the treatment vessel in a flow path. b) Selectively at least one of deflecting, substantially unencumbering, or substantially preventing, the flow from a) at a plurality of positions around the flow path so as to cause the material to establish a relatively flat top profile in the treatment vessel so that the material will be more uniformly treated in the treatment vessel, or have the treatment time thereof extended, than if a non-relatively flat, for example, sharply conical top profile were established. And, c) substantially uniformly treating the material in the treatment vessel.
The method may be practiced utilizing a plurality of gates disposed around the flow path, and b) may be practiced by individually moving the gates to partially or substantially fully open, or partially or substantially fully closed, all or selected portions of the flow path. Typically b) is further practiced by automatically individually moving the gates, typically by moving at least three polygonal shaped gates, such as individually moving four triangular shaped gates. The method may be practiced utilizing a fluid controlled piston and cylinder assembly attached to each gate, in which case b) is further practiced by controlling the supply of pressurized fluid to the piston and cylinder assemblies to individually control the positions of the gates attached thereto.
Typically c) is practiced by steaming the material in the pile in the treatment vessel, and b) is typically practiced at least in part in response to sensing of the level of material in the treatment vessel, such as by sensing the top profile of material in the treatment vessel. Where the isolation device is a distinct vessel above the treatment vessel, b) may be practiced in part in response to sensing of the level of material in the isolation device. For example b) is practiced by controlling the gates so that they open and close for a predetermined period of time ranging between about one second and ten minutes, typically between about five seconds and five minutes, for example between about fifteen seconds and one minutes, in a predetermined sequence.
According to another aspect of the present invention there is provided a system for treating comminuted cellulosic fibrous material, comprising: A treatment vessel having a top and a bottom. An inlet for comminuted cellulosic fibrous material at or adjacent the top, typically including an isolation device. A plurality of individually controlled gates each movable between a position in which it is substantially completely closed and a position in which it is substantially fully open, or to at least some positions between these extremes (e.g. substantially any position between fully open and closed). The gates mounted below the inlet, so as to deflect, not significantly affect, or substantially preclude the flow of material therepast into the vessel, depending upon the positions thereof. And, an individual actuator for each of the gates.
The plurality of gates typically comprises at least three substantially polygonal shaped gates, such as four substantially triangular shaped gates. The actuators may comprise pneumatic or hydraulic piston and cylinder assemblies, and there may further be an automatically controlled valve operatively connected to each of the actuators that control the flow of pressurized fluid thereto. A sensor may be operatively connected to the valves for controlling the operation thereof, and/or the valves may be controlled by a timer. The sensor may comprise a level sensor for sensing the level of material in the treatment vessel; for example, the sensor may comprise a plurality of sensors disposed around the treatment vessel for sensing the top profile of a pile of material in the treatment vessel.
The isolation device may comprise a distinct vessel on top of the treatment vessel, and the system may further comprise a level sensor for sensing the level of material in the isolation device; a pressurized fluid supply tank operatively connected to the valves; and a flow control valve operatively connected to the supply tank and controlled in response to sensing of material level in the isolation device sensed by the level sensor. The treatment vessel typically includes treatment fluid introducing structures, preferably a treatment vessel comprising a chip bin and including at least one steam introduction device which introduces steam into the treatment vessel to steam the material therein.
According to another aspect of the present invention there is provided a system for treating comminuted cellulosic fibrous material, comprising: A treatment vessel having a top and a bottom. An inlet for comminuted cellulosic fibrous material at or adjacent the top. A plurality of individually controlled gates each movable and positionable between a position in which it is substantially completely closed and a position in which it is substantially fully open or to positions between these extremes (e.g. any position between fully open and closed). The gates mounted below the inlet, so as to deflect, not significantly affect, or substantially preclude the flow of material therepast into the vessel, depending upon the positions thereof. Means for selectively moving the gates (such as conventional individual mechanical, fluidic, and/or electric elements operatively connected to each gate) so as to cause the material to establish a relatively flat top profile in the treatment vessel so that the material will be more uniformly treated in the treatment vessel than if a non-relatively flat, for example, sharply conical, top profile were established. And, means for introducing treatment fluid (such as conventional steam nozzles, bars, or grids, or liquid spray heads or conduits or nozzles) into the treatment vessel so as to substantially uniformly treat the material in the treatment vessel. The inlet may include a means for isolating the interior of the vessel from the atmosphere, for example, an isolation device.
Thus the present invention provides an improved method and apparatus which distribute wood chips (or other comminuted cellulosic fibrous material) across the top of a retention vessel (e. g., chip bin) to improve the uniformity of the treatment or extend the time of the treatment in the vessel. This invention is typically applicable to the treatment of wood chips during their introduction to a chemical pulping process, but is applicable to any treatment or retention of any particulate matter (including non-cellulose particulate matter) for which better distribution in a vessel is desirable.